The generation of large amplitude, relativistic plasma waves is a subject of much current interest because of its potential use for ultrahigh-gradient electron acceleration.
There are two other major types of laser driven, plasma based accelerators: the plasma beatwave accelerator (PBWA) and the laser wakefield accelerator (LWFA). While advanced RF-driven accelerators are limited to fields .ltoreq.1 MV/cm, plasma accelerators have been shown experimentally to support gradients .about.10 MV/cm. The maximum axial electric field of a relativistic plasma wave, as predicated by 1-D cold fluid theory, is the "wave breaking" field: E.sub.WB =(m.sub.e cw.sub.p /e).sqroot.2(.gamma..sub.p -1), which can exceed 1 GV/cm, where w.sub.p =(4.pi..sup.2n.sub.e0 /m.sub.e).sup.1/2 is the electron plasma frequency, n.sub.e0 is the ambient electron density, .gamma..sub.p =(1-v.sub.p.sup.2 /c.sup.2).sup.-1/2 and v.sub.p is the phase velocity of the plasma wave.
In the PBWA, two laser beams of frequencies w and w-w.sub.p are optically mixed in a plasma to produce a laser beatwave, in effect a train of equally spaced pulses, which "resonantly" excites a large amplitude plasma wave. A fundamental limitation to the plasma wave amplitude in the PBWA is resonant detuning. As the plasma wave amplitude grows, nonlinear effects cause the resonant frequency to shift away from w.sub.p, which leads to saturation and thus limits the plasma wave amplitude. In the LWFA, a single, intense, short laser pulse drives a plasma wave "wakefield". The maximum plasma wave amplitude results when .tau..about.2 .pi./w.sub.p, where .tau. is the laser pulse width, which translates into a "resonant density", since w.sub.p .about.n.sub.e0.sup.1/2. Recently, the self-modulated LWFA has been suggested. Here, a single laser pulse is incident on a plasma with a density that is higher than the "resonant density". Due to a self-modulation instability, the pulse breaks up into multiple pulses, each of which are "resonant". Although higher plasma densities and the multiple pulse structure lead to higher wakefield amplitudes, both high plasma densities and high laser intensities are difficult to achieve simultaneously due to plasma defocusing, and electron acceleration is limited by phase detuning, i.e., accelerated electrons (with v.fwdarw.c) outrun the plasma wave (with v.sub.p .apprxeq.v.sub.g &lt;c).